Nonwoven article and method of making the same

ABSTRACT

A method comprises exposing a particle coating disposed on a nonwoven fiber web comprising thermally-softenable fibers to pulsed electromagnetic radiation having at least one wavelength in the range of 200 nm to 1000 nm. The particle coating comprises distinct particles that are not chemically bonded to each other, and are not retained in a binder material other than the thermally-softenable fibers. Also disclosed are nonwoven articles comprising a thermally-softenable nonwoven fiber web having a particle coating disposed thereon. The particle coating comprises distinct particles that are not chemically bonded to each other and are not retained in a binder material other than the thermally-softenable nonwoven fiber web. The particle coating is at least 60 percent retained after a one minute immersion in isopropanol at 22° C.

TECHNICAL FIELD

The present disclosure broadly relates to methods for improving thedurability of particle coatings on nonwoven fiber webs, and articlespreparable thereby.

BACKGROUND

Coatings of powders (e.g., graphite) on nonwoven fiber webs are widelyknown; however, the powders are typically loosely bound to the fibersand are prone to falling off. Various methods have been devised toovercome this problem, including: 1) use of a curable resin applied tothe fibers prior to powder coating, and that when cured securely bindsthe powder to the fibers; 2) in those cases where the nonwoven fiber webis durable enough, the powder may be rubbed onto it in a process knownas triboadhesion; and 3) the powders can be selected to contain bindercomponents that can fuse to the fibers on heating.

However, each of these techniques has disadvantages if a particlecoating consisting essentially of inorganic particles is desired. Forexample, the presence of binder components in approaches 1) and 3) wouldbe unacceptable in such a situation, and durability of particle coatingsmade by approach 2) is generally problematic as particle coatings aretypically prone to damage by methods such as abrasion and/or rinsingwith solvent.

SUMMARY

Advantageously, the present disclosure provides an easy method toenhance the durability of particle coatings that involves instantaneousheating by exposure to pulsed electromagnetic radiation having at leastone wavelength in the range of 200 nm to 1000 nm. Without wishing to bebound by theory, the present inventors believe that the modulatedelectromagnetic radiation hitting the particles in the particle coatingis converted to heat that is localized adjacent to the particles therebysoftening the adjacent fibers and increasing adhesion between thosefibers and the particles.

In a first aspect, the present disclosure provides a method of making anonwoven article, the method comprising exposing a particle coatingdisposed on a thermally-softenable nonwoven fiber web to pulsedelectromagnetic radiation having at least one wavelength in the range of200 to 1000 nanometers, wherein the particle coating comprises looselybound distinct particles that are not chemically bonded to each otherand are not retained in a binder material other than thethermally-softenable nonwoven fiber web, and wherein the pulsedelectromagnetic radiation has sufficient fluence and pulse width toincrease bonding force between at least a portion of the loosely bounddistinct particles and the thermally-softenable nonwoven fiber web.

By this technique, durability of the particle coating is improved, whilealternative heating methods were prone to damaging (e.g., warping) thethermally-softenable nonwoven fiber web.

Accordingly, in a second aspect, the present disclosure provides anonwoven article made according to the foregoing method of the presentdisclosure.

In a third aspect, the present disclosure provides a nonwoven articlecomprising a thermally-softenable nonwoven fiber web having a particlecoating disposed thereon, wherein the particle coating comprisesdistinct particles that are not chemically bonded to each other and arenot retained in a binder material other than the thermally-softenablenonwoven fiber web, and wherein the particle coating is at least 60percent retained after a one minute immersion in isopropanol at 22° C.

As used herein:

The term “visible light” refers to electromagnetic radiation having awavelength of 400 to 700 nanometers (nm).

The term “powder” refers to a free-flowing collection of minuteparticles.

The term “pulsed electromagnetic radiation” refers to electromagneticradiation that is modulated to become a series of discrete spikes withincreased intensity. The spikes may be relative to a background level ofelectromagnetic radiation that is negligible or zero, or the backgroundlevel may be at a higher level that is substantially ineffective toincrease adhesion of particles in the particle coating to the fiber.

The term “thermally-softenable” means softenable upon heating.

The term “particle coating” refers to a coating of minute particleswhich may or may not be free-flowing.

Features and advantages of the present disclosure will be furtherunderstood upon consideration of the detailed description as well as theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged schematic side view of an exemplary article 100according to the present disclosure.

It should be understood that numerous other modifications andembodiments can be devised by those skilled in the art, which fallwithin the scope and spirit of the principles of the disclosure.

DETAILED DESCRIPTION

Advantageously, the present disclosure provides an easy method toenhance the durability of particle coatings on nonwoven fiber webs usinginstantaneous heating by exposure to a modulated source ofelectromagnetic radiation.

Referring now to FIG. 1, exemplary article 100 comprises athermally-softenable nonwoven fiber web 110 having a particle coating120 disposed thereon.

Particle coatings on thermally-softenable nonwoven (e.g., thermoplastic)fiber webs can be carried out by various known methods including, forexample, exposure to an aerosolized particle cloud, contact with apowder bed, coating with a solvent-based particle dispersion coatingfollowed by evaporation of solvent, and/or powder-rubbed (rubbing dryparticles against a substrate to form a coating of the powderparticles). Examples of powder-rubbing methods can be found in U.S. Pat.No. 6,511,701 B1 (Divigalpitiya et al.), U.S. Pat. No. 6,025,014(Stango), and U.S. Pat. No. 4,741,918 (Nagybaczon et al.). The remainingmethods will be familiar to those of ordinary skill in the art.

Useful particle coatings comprise minute loosely bound particles capableof absorbing at least one wavelength of the pulsed electromagneticradiation, preferably corresponding to a majority of the energy of thepulsed electromagnetic radiation. Suitable particles are preferably atleast substantially unaffected by electromagnetic radiation, but aremoderate to strong absorbers of it. This is desirable to maximize thelight (electromagnetic radiation) to heat conversion yield withoutaltering the chemical nature of the particles.

Exemplary suitable particles include graphite, clays, hexagonal boronnitride, pigments, inorganic oxides (e.g., alumina, calcia, silica,ceria, zinc oxide, or titania), metal(s), organic polymeric particles(e.g., polytetrafluoroethylene, polyvinylidene difluoride), carbides(e.g., silicon carbide), flame retardants (e.g., aluminum trihydrate,aluminum hydroxide, magnesium hydroxide, sodium hexametaphosphate,organic phosphonates and phosphates and ester thereof), carbonates(e.g., calcium carbonate, magnesium carbonate, sodium carbonate), drybiological powders (e.g., spores, bacteria), and combinations thereof.Preferably, the particles have an average particle size of 0.1 to 100micrometers, more preferably 1 to 50 micrometers, and more preferably 1to 25 micrometers, although this is not a requirement. Graphite andhexagonal boron nitride are particularly preferred in many applications

Prior to exposure to the electromagnetic radiation the particle coatingcomprises loosely bound distinct particles that are not chemicallybonded to each other, and are not retained in a binder material otherthan the thermally-softenable nonwoven fiber web itself.

The thermally-softenable nonwoven fiber web preferably comprisesthermoplastic fibers, although non-thermoplastic fibers may be usedalone or in combination with thermoplastic fibers, for example. Inpreferred embodiments, the fibers of the thermally-softenable nonwovenfiber web are non-tacky and/or non-thermosetting, although this is not arequirement.

Exemplary suitable thermally-softenable nonwoven fiber webs includemeltspun fiber webs, blown microfiber webs, needletacked staple fiberwebs, thermally bonded airlaid webs, and spunlace webs. Thethermally-softenable nonwoven fiber web may be made by any suitablenonwoven fiber web making process. Examples include meltspun, blownmicrofiber (BMF), air-laid processes, wet-laid processes, and spunlace.These and other methods will be known to those of skill in the art.Alternatively, a wide array of nonwoven fiber web comprisingthermally-softenable fibers are commercially available. Thethermally-softenable nonwoven fiber web may be of any basis weight andmay be densely compacted or lofty and open, for example.

Some examples of thermoplastic polymers that may be suitable forfiber-forming include polycarbonates, polyesters, polyamides,polyurethanes, polyacrylics (e.g., polyacrylonitrile), block copolymerssuch as styrene-butadiene-styrene and styrene-isoprene-styrene blockcopolymers, polyolefins such as polyethylene, polypropylene,polybutylene, and poly(4-methyl-1-pentene), and combinations of suchresins. Examples of thermoplastic polymers materials that may be used tomake nonwoven fiber web comprising thermoplastic fibers are disclosed inU.S. Pat. No. 5,706,804 (Baumann et al.), U.S. Pat. No. 4,419,993(Peterson), Re 28,102 (Mayhew), U.S. Pat. No. 5,472,481 (Jones et al.),U.S. Pat. No. 5,411,576 (Jones et al.), and U.S. Pat. No. 5,908,598(Rousseau et al.). In some preferred methods, at least a portion of thefibers in the thermoplastic fiber web have a higher melting core and alower melting sheath. In such cases, the higher melting core shouldpreferably be at least 25° C.

The pulsed electromagnetic radiation may come from any source(s) capableof generating sufficient fluence and pulse duration to effect sufficientheating of the nonwoven fiber web to cause the particle coating to bindmore tightly to it. At least three types of sources may be effective forthis purpose: flashlamps, lasers, and shuttered lamps. The selection ofappropriate sources will typically be influenced by desired processconditions such as, for example, line speed, line width, spectraloutput, and cost.

Preferably, the pulsed electromagnetic radiation is generated using aflashlamp. Of these, xenon and krypton flashlamps are the most common.Both provide a broad continuous output over the wavelength range 200 to1000 nanometers, however the krypton flashlamps have higher relativeoutput intensity in the 750-900 nm wavelength range as compared to xenonflashlamps which have more relative output in the 300 to 750 nmwavelength range. In general, xenon flashlamps are preferred for mostapplications, and especially those involving graphite particles. Manysuitable xenon and krypton flashlamps are commercially available fromvendors such as Excelitas Technologies Corp. of Waltham, Mass. andHeraeus of Hanau, Germany.

In another embodiment, the pulsed electromagnetic radiation can begenerated using a pulsed laser. Suitable lasers may include, forexample, excimer lasers (e.g., XeF (351 nm), XeCl (308 nm), and KrF (248nm)), solid state lasers (e.g., ruby 694 nm)), and nitrogen lasers(337.1 nm).

In yet another embodiment, the pulsed electromagnetic radiation isgenerated using a continuous light source and a shutter (preferably arotating aperture/shutter to reduce overheating of the shutter).Suitable light sources may include high-pressure mercury lamps, xenonlamps, and metal-halide lamps.

For maximum efficiency, the electromagnetic radiation spectrum ispreferably most intense at wavelength(s) that are strongly absorbed bythe particles, although this is not a requirement. Likewise, in the caseof reflective particles, the electromagnetic radiation spectrum ispreferably most intense in spectral regions in which the particles areleast reflective, although this is not a requirement.

Preferably, the source of pulsed electromagnetic radiation is capable ofgenerating a high fluence (energy density) with high intensity (highpower per unit area), although this is not a requirement. Theseconditions assure that the sufficient heat is absorbed to effectincreased adhesion of the particles to the fibers. However, thecombination of intensity and fluence should not be so great/high as tocause ablation, excessive degradation, or volatilization of fibers inthe nonwoven fiber web. Selection of appropriate conditions is withinthe capability of one of ordinary skill in the art.

To minimize heating of interior portions of the fibers that cannotinteract with the particles on the nonwoven fiber web, the pulseduration is preferably short; e.g., less than 10 milliseconds, less than1 millisecond, less than 100 microseconds, less than 10 microseconds, oreven less than 1 microsecond, although this is not a requirement.

To achieve high line speed in continuous manufacturing processes, notonly should the pulsed electromagnetic radiation preferably be powerful,but the exposure area is preferably large and the pulse repetition rateis preferably fast (e.g., 100 to 500 Hz).

In order to evaluate durability, the resultant exposed particle-coatednonwoven fiber web may be immersed in a solvent such as, e.g.,isopropanol for a fixed interval (e.g., 1, 2, 3, 4, or even 5 minutes,or longer) at about 22° C. (e.g., room temperature), and then removed,dried, and weighed. Weight loss of powder can then be determined bysubtraction. The solvent should be selected such that it does notdissolve the nonwoven fiber web.

Preferably, the particulate coating of the nonwoven article issufficiently bonded to the nonwoven fiber web so that after one minuteof immersion in isopropanol at 22° C. at least 15, 20, 25, 30, 35, 40,45, 50, 55, 60, 65, 70, 75, 80, 85, or even at least 90 percent of theparticulate coating remains bonded to the nonwoven fiber web.

SELECT EMBODIMENTS OF THE PRESENT DISCLOSURE

In a first embodiment, the present disclosure provides a method ofmaking a nonwoven article, the method comprising exposing a particlecoating disposed on a thermally-softenable nonwoven fiber web to pulsedelectromagnetic radiation having at least one wavelength in the range of200 to 1000 nanometers, wherein the particle coating comprises looselybound distinct particles that are not chemically bonded to each otherand are not retained in a binder material other than thethermally-softenable nonwoven fiber web, and wherein the pulsedelectromagnetic radiation has sufficient fluence and pulse width toincrease bonding force between at least a portion of the loosely bounddistinct particles and the thermally-softenable nonwoven fiber web.

In a second embodiment, the present disclosure provides a methodaccording to the first embodiment, wherein the particle coatingcomprises at least one of graphite or hexagonal boron nitride.

In a third embodiment, the present disclosure provides a methodaccording to the first or second embodiment, wherein the particlecoating consists essentially of graphite.

In a fourth embodiment, the present disclosure provides a methodaccording to any one of the first to third embodiments, wherein thepulsed electromagnetic radiation is generated using a flashlamp.

In a fifth embodiment, the present disclosure provides a methodaccording to any one of the first to third embodiments, wherein thepulsed electromagnetic radiation is generated using a pulsed laser.

In a sixth embodiment, the present disclosure provides a methodaccording to any one of the first to third embodiments, wherein thepulsed electromagnetic radiation is generated using a continuous lightsource and a shutter.

In a seventh embodiment, the present disclosure provides a methodaccording to any one of the first to sixth embodiments, wherein thethermally-softenable nonwoven fiber web comprises fibers having a highermelting core and a lower melting sheath.

In an eighth embodiment, the present disclosure provides a nonwovenarticle made according to any one of the first to seventh embodiments ofthe present disclosure.

In a ninth embodiment, the present disclosure provides a nonwovenarticle comprising a thermally-softenable nonwoven fiber web having aparticle coating disposed thereon, wherein the particle coatingcomprises distinct particles that are not chemically bonded to eachother and are not retained in a binder material other than thethermally-softenable nonwoven fiber web, and wherein the particlecoating is at least 60 percent retained after a one minute immersion inisopropanol at 22° C.

In a tenth embodiment, the present disclosure provides a nonwovenarticle according to the ninth embodiment, wherein the particle coatingcomprises at least one of graphite or hexagonal boron nitride.

In an eleventh embodiment, the present disclosure provides a nonwovenarticle according to the ninth or tenth embodiment, wherein the particlecoating consists essentially of graphite.

In a twelfth embodiment, the present disclosure provides a nonwovenarticle according to any one of the ninth to eleventh embodiments,wherein the particle coating is at least 90 percent retained after theone minute immersion in isopropanol at 22° C.

In a thirteenth embodiment, the present disclosure provides a nonwovenarticle according to any one of the ninth to twelfth embodiments,wherein the thermally-softenable nonwoven fiber web comprises fibershaving a higher melting core and a lower melting sheath.

Objects and advantages of this disclosure are further illustrated by thefollowing non-limiting examples, but the particular materials andamounts thereof recited in these examples, as well as other conditionsand details, should not be construed to unduly limit this disclosure.

EXAMPLES

Unless otherwise noted, all parts, percentages, ratios, etc. in theExamples and the rest of the specification are by weight. All reagentsused in the examples were obtained, or are available, from generalchemical suppliers such as, for example, Sigma-Aldrich Company, SaintLouis, Missouri, or may be synthesized by conventional methods.

Materials used in the Examples

DESIGNATION DESCRIPTION PE nonwoven Meltblown linear low densitypolyethylene nonwoven fiber web, basis weight = 63 g/m², 0.33 mmthickness MICRO 850 Graphite powder, 3-5 micrometer particle size, 13m²/g surface area, 0.088 ohm · cm resistivity, obtained from AshburyGraphite Mills, Inc., Kittanning, Pennsylvania as MICR0850 Isopropanol(IPA) solvent, obtained from Aldrich Chemical Company Milwaukee,Wisconsin

General Method for Coating Graphite onto Substrates

To make Examples and Comparative Examples described below, graphitecoatings were applied on PE nonwoven substrates by placing a strip ofnonwoven approximately 1.5 inches (3.8 cm) by 10 inches (25.4 cm) indimension and a small amount of MICRO850 in a sealable plastic bag. Thebag was then sealed and shaken, until the PE nonwoven was visiblycovered in graphite. The nonwoven was then removed, and excess graphiteparticles were removed by blowing with compressed nitrogen at a pressureof 40 pounds per square inch.

The relative amount of graphite coating deposited on the PE nonwovenfilm was determined by measuring the weight of the sample before andafter the process.

General Methods for Determining Durability

The samples prepared according to Examples and Comparative Examplesdescribed below, were tested for durability (resilience of coatings).

Nonwoven samples were completely immersed (i.e., submerged) in a bath ofIPA at room temperature (22° C.) and stirred by hand for 1 minute. Thesamples were then removed and spread onto a clean surface in a chemicalhood and allowed to dry completely.

All reported percentages of graphite retained (% R) were calculationsfrom the following equation:

${\% \mspace{14mu} R}\; = \; {100 \times \frac{M_{g,i} - M_{g,w}}{M_{g,i}}}$

Where M_(g,i) is the mass of graphite on the nonwoven just prior toimmersion in isopropanol, and M_(g,w) is the mass of graphite remainingon the nonwoven after the wash step.

Examples 1-11 (EX-1 to EX-11) and Comparative Example A (CEX-A)

CEX-A and EX-1 to EX-12 were graphite coated PE nonwoven substratesprepared as described above. For CEX-A, the substrate was not subjectedto IPL and was a control sample. EX-1 to EX-11 were prepared bysubjecting the samples to an intense pulsed light irradiation (IPL). Inall cases of IPL, the source used was a Xe flashlamp, commerciallyobtained from Xenon Corporation, Wilmington, Mass., as a SINTERON S-2100Xe flashlamp equipped with Type C bulb. Samples were placed beneath aquartz plate for the irradiation process.

For EX-1, the substrate was treated 1 time at a pulse rate of 1 Hz andan energy density of 0.1 J/cm². The substrate was then removed andflipped over, and the treatment was repeated on the backside of thesubstrate.

For EX-2, the substrate was treated 3 times at a pulse rate of 1 Hz andan energy density of 0.1 J/cm². The substrate was then removed andflipped over, and the treatment was repeated on the backside of thesubstrate.

For EX-3, the substrate was treated 5 times at a pulse rate of 1 Hz andan energy density of 0.1 J/cm². The substrate was then removed andflipped over, and the treatment was repeated on the backside of thesubstrate.

For EX-4, the substrate was treated 1 time at a pulse rate of 1 Hz andan energy density of 0.2 J/cm². The substrate was then removed andflipped over, and the treatment was repeated on the backside of thesubstrate.

For EX-5, the substrate was treated 3 times at a pulse rate of 1 Hz andan energy density of 0.2

J/cm². The substrate was then removed and flipped over, and thetreatment was repeated on the backside of the substrate.

For EX-6, the substrate was treated 5 times at a pulse rate of 1 Hz andan energy density of 0.2 J/cm². The substrate was then removed andflipped over, and the treatment was repeated on the backside of thesubstrate.

For EX-7, the substrate was treated 1 time at a pulse rate of 1 Hz andan energy density of 0.3 J/cm². The substrate was then removed andflipped over, and the treatment was repeated on the backside of thesubstrate.

For EX-8, the substrate was treated 3 times at a pulse rate of 1 Hz andan energy density of 0.3 J/cm². The substrate was then removed andflipped over, and the treatment was repeated on the backside of thesubstrate.

For EX-9, the substrate was treated 5 times at a pulse rate of 1 Hz andan energy density of 0.3 J/cm². The substrate was then removed andflipped over, and the treatment was repeated on the backside of thesubstrate.

For EX-10, the substrate was treated 1 time at a pulse rate of 1 Hz andan energy density of 0.4

J/cm². The substrate was then removed and flipped over, and thetreatment was repeated on the backside of the substrate.

For EX-11, the substrate was treated 3 times at a pulse rate of 1 Hz andan energy density of 0.4 J/cm². The substrate was then removed andflipped over, and the treatment was repeated on the backside of thesubstrate.

Table 1, below, reports the IPL effects on PE nonwoven and the measuredfraction (f) of the graphite coating retained.

TABLE 1 ENERGY DENSITY IPL PULSES PER PULSE, EXAMPLE PER SIDE J/cm² %RCEX-A 0 0 4.9 EX-1 1 0.1 16.6 EX-2 3 0.1 26.8 EX-3 5 0.1 31.5 EX-4 1 0.218.6 EX-5 3 0.2 44.0 EX-6 5 0.2 45.4 EX-7 1 0.3 16.5 EX-8 3 0.3 52.3EX-9 5 0.3 60.5 EX-10 1 0.4 34.8 EX-11 3 0.4 91.0

Comparative Examples B-D (CEX-B to CEX-D)

For CEX-B to CEX-D, samples of nonwoven PE were subjected to heating ina Model 725G Isotemp laboratory oven (Fisher Scientific, Hampton, NewHampshire). Samples were placed on an aluminum tray in a preheated ovenfor the specified amount of time. Results are reported in Table 2,below.

TABLE 2 TEMPERATURE, DURATION, EXAMPLE ° C. minutes %R CEX-B 105 5 27.5CEX-C 105 15 37.0 CEX-D 105 30 54.8

All cited references, patents, and patent applications in the aboveapplication for letters patent are herein incorporated by reference intheir entirety in a consistent manner. In the event of inconsistenciesor contradictions between portions of the incorporated references andthis application, the information in the preceding description shallcontrol. The preceding description, given in order to enable one ofordinary skill in the art to practice the claimed disclosure, is not tobe construed as limiting the scope of the disclosure, which is definedby the claims and all equivalents thereto.

1-13. (canceled)
 14. A method of making a nonwoven article, the method comprising exposing a particle coating disposed on a thermally-softenable nonwoven fiber web to pulsed electromagnetic radiation having at least one wavelength in the range of 200 to 1000 nanometers, wherein the particle coating comprises loosely bound distinct particles that are not chemically bonded to each other and are not retained in a binder material other than the thermally-softenable nonwoven fiber web, and wherein the pulsed electromagnetic radiation has sufficient fluence and pulse width to increase bonding force between at least a portion of the loosely bound distinct particles and the thermally-softenable nonwoven fiber web.
 15. The method of claim 14 wherein the particle coating comprises at least one of graphite or hexagonal boron nitride.
 16. The method of claim 14, wherein the particle coating consists essentially of graphite.
 17. The method of claim 14, wherein the pulsed electromagnetic radiation is generated using a flashlamp.
 18. The method of claim 14, wherein the pulsed electromagnetic radiation is generated using a pulsed laser.
 19. The method of claim 14, wherein the pulsed electromagnetic radiation is generated using a continuous light source and a shutter.
 20. The method of claim 14, wherein the thermally-softenable nonwoven fiber web comprises fibers having a higher melting core and a lower melting sheath.
 21. A nonwoven article made according to the method of claim
 1. 22. A nonwoven article comprising a thermally-softenable nonwoven fiber web having a particle coating disposed thereon, wherein the particle coating comprises distinct particles that are not chemically bonded to each other and are not retained in a binder material other than the thermally-softenable nonwoven fiber web, and wherein the particle coating is at least 60 percent retained after a one minute immersion in isopropanol at 22° C.
 23. The nonwoven article of claim 22, wherein the particle coating comprises at least one of graphite or hexagonal boron nitride.
 24. The nonwoven article of claim 22, wherein the particle coating consists essentially of graphite.
 25. The nonwoven article of claim 22, wherein the particle coating is at least 90 percent retained after the one minute immersion in isopropanol at 22° C.
 26. The nonwoven article of claim 22, wherein the thermally-softenable nonwoven fiber web comprises fibers having a higher melting core and a lower melting sheath. 